Don't worry, this picture's presence will make sense by the end. I think.

Two years ago, I wrote this post on running back aging curves. One conclusion from my research was that age 26 was the peak age for running backs, which was immediately followed by a steady decline phase until retirement. In that study, I only wanted to look at very good-to-excellent running backs in the modern era; as a result, I was forced to limit myself to just 36 players. I’ve been meaning to update that post, but wasn’t quite sure what methodology to use.

Last year, Neil wrote a very interesting post on quarterback aging curves. In it, Neil computed the year-to-year differences in Relative ANY/A at every age. While reviewing that post, a lightbulb went off. We can greatly increase the sample size if we only look at running backs from year-to-year, and not just the best running backs on the career level.

There are 723 running backs since 1970 who had at least 150 carries in consecutive seasons and who were between 21 and 32 in the first of those two seasons. For each running back pair of seasons, I calculated how many rushing yards the player gained in Year N and many yards he gained in Year N+1. Take a look:

Age

#

Rsh Yds Yr N

Rsh Yds N+1

Diff

21

16

989

1078

89

22

43

1057

986

-70

23

92

993

1067

74

24

115

1046

1017

-29

25

124

1043

1069

26

26

107

1121

1070

-51

27

83

1105

1007

-98

28

59

1067

994

-74

29

37

1081

982

-100

30

26

1118

1008

-110

31

13

1104

906

-198

32

8

1006

769

-237

Just so we’re clear on what that table says, let me walk through an example. There were 43 running backs who were 22-years-old and had at least 150 carries, and also had at least 150 carries at age 23.1 Those running backs averaged 1,057 yards at age 22 and then 986 yards at age 23. While these numbers may be useful for reference purposes, if you’re like me, a table like this isn’t particularly intuitive; a graph would be much better at explaining what the data tell us.

If we assume that a running back will rush for 1,000 yards at age 21, and then gain or lose the amount of yards showed in the table above each season (i.e., will rush for 89 more yards at age 22, then 70 fewer yards at age 23, then 74 more yards at age 24, and so on), he would produce the following career curve:

The dotted blue line tracks our fictional player who rushed for 1,000 yards at age 21. But the smoothed black line is probably the more useful one, which is presumably a better representation of the effects of age on a running back’s production. What’s interesting to me is that despite using several different variables and measuresh, this study comes to a pretty similar conclusion to the last one: running backs peak at age 26, and then begin a steady decline. It also suggests that for older running backs, some dropoff should always be expected from year-to-year, even if they have done very well in the prior season.

Do you know which running back’s career most closely resembles our set of averages? The answer: Buffalo Bills Hall of Famer Thurman Thomas. The graph below shows Thomas’s yearly rushing averages from ages 22 (when he entered the league) through 33: as you can see, the curve is shifted upwards because he was better than the average back, but the shape of the curves are pretty similar. If nothing else, may you remember that Thurman Thomas aged like your typical running back.

Thomas had a great five-year peak from ages 23-to-27 on which his Hall of Fame career was built. That was followed by a respectable but inferior three-year period; after that, his production fell off a cliff sharply, beginning after his age 30 season.

What can we take from today’s post? The survivorship issue2 has not been resolved, although I don’t think it’s a huge issue here. If anything, my guess would be that it understates the degree of magnitude by which older running backs decline. Another conclusion is that running backs don’t take very long to progress in their careers: a running back in the draft is not far behind, if at all, from a running back who is 25 or 26.3 This is your 1,283rd reminder that giving big money contracts to older running backs is one of the riskiest moves an organization can make. Three years ago, a 26-year-old Maurice Jones-Drew led the league in rushing. Over a week into free agency, the 29-year-old running back is still looking for a job. Last season, a 26-year-old Knowshon Moreno gained 1,586 yards from scrimmage and scored 13 touchdowns. He’s still looking for a job, too.

Update: For those curious about the choppiness at the beginning of the curve, the table below shows the results for the 22-year-old running backs:

Careful readers will quickly recognize that this opens us up survivorship bias issues. Running backs who decline greatly from Year N to Year N+1 won’t have their failures recognized in this study if they fail to hit the 150-carry threshold. Unfortunately, I can’t quite think of a good solution right now, but perhaps one will come to me (via the comments?) by the time Part III is produced. [↩]

Of course, we’re focused just on production in the running game here, not things like pass protection. [↩]

Sunrise089

Missing word: “of you’re like me, a table like this ISN’T particularly intuitive.”

Also, what’s with the choppy curve early? I’d think 40+ seasons would be enough to smooth it out. Is there anything weird going on with draft age versus skill?

Chase Stuart

Fixed.

I posted the data on all the 22-year-old backs. I think the choppiness there is almost entirely a function of injury: as you can see in the average row at the end, the numbers are pretty similar on a per-game basis. I thought about using per-game numbers for all backs (while keeping the 150-carry cutoff) and then prorating every year to 16 games, but I didn’t since risk of injury is one of the things people are focused on with older running backs. Of course, maybe this is just a reminder that any running back can get hurt.

I’m sure there are exceptions, I’d argue modern day exception would be Matt forte ?

Nate

Does the chart look any different if you use yards per carry instead of total yards?

http://nflsgreatest.co.nf/ Laverneus Dinglefoot

I know you don’t want to include pass blocking, but what about including receiving production and using scrimmage yards from n to n+1? That may be a better way of describing usage rates, and expanding the threshold to 150 offensive touches would give you even more players to work with (then again, you’d have guys like Larry Centers who confuse things). I think a lot of backs have a similar dropoff in receiving production, but there are probably exceptions.

Chase Stuart

My guess is that little changes, but a good idea for a future post.

Richie

Two years ago, I wrote this post on running back aging curves.

Can’t believe this site has already been up for 2+ years.

Chase Stuart

Me too! Although the two year anniversary comes in June. I believe you lead the way with comments here, by the way!

Tim Truemper

Two thoughts: What if total yards from scrimmage was used rather than just rushing yards; and what RB’s buck the trend, i.e. that they had relatively strong performance after age 30 when compared to their peak years.

Chase Stuart

My guess is the numbers would be very similar if total yards from scrimmage were used. As for the other question, I think Part I answers that one pretty well.

Richie

RB’s have recently come to be considered a position that is easy to fill by just about anybody. I always figured this is because so much of a RB’s success hinges on the offensive line opening up holes for him.

But if it’s true that RB’s are heavily reliant on their offensive lines, wouldn’t that mean that RB production wouldn’t drop off so quickly and consistently as they age? After all, how much “worse” is a RB really going to get between ages 29 and 32, especially if his line is really doing most of the work?

Chase Stuart

That’s a good point. I hadn’t thought of it like that before.

Richie

Neither had I.

Thinking about it some more. What skills do good RB’s have?

– Vision. The ability to quickly spot holes in the defense, and tacklers coming their way. I wouldn’t think this is a skill that is lost at such an early age. I would think this is something that could last until a player is 40 or 50.
– Juke. The ability to make quick changes to go where the defenders ain’t. This seems like the main skill that would deteriorate. It probably doesn’t take much difference in juke ability to explain the difference between a “good” RB and an All-Pro.
– Power. The ability to break tackles and/or run over defenders. This is another skill that probably doesn’t deteriorate as quickly. Some of those strong man and physical fitness competitions have guys who are in their 40s. I think guys can keep most of their strength into their 40s or 50s.
– Speed. The ability to outrun defenders. (As opposed to the quicker “juke” ability that is just about moving very short distances quickly.) Speed is something else that players probably lose at an early age. But I don’t think this is a critical skill for a RB. Yeah, it’s nice if a RB can outrun some defenders, but most of the true success of a RB comes from the ability to tack on a bunch of 5 to 10 yard runs over the course of a game or season. Though I would be really interested to see some sort of distribution chart of length of runs for good RB’s over the course of a season. Maybe a guy really does need to have about 10 runs per year that go for 20 yards or more in order to put up a 1,200+ rushing yard season.

sn0mm1s

Speed is very important in regards to hitting a hole before it closes. I forget who interviewed Sanders but he asked (and I paraphrase) “What is it that you see that no one else does?” Sanders’ reply “I see the same thing you do – I can just get there.”

Also don’t forget recovery time. It takes longer and longer to recover from the pounding a RB gets each game.

Rob Harrison

Good points, but I think you’re wrong w/r/t power. From what I can see, I think power effectively deteriorates quickly, not because RBs lose strength with age, but because the more a back runs over/through defenders, the more beaten up he gets and the more injuries he accumulates.

sn0mm1s

Oline is likely only important for the first few yards. The great RBs need less of an Oline and when they do get a hole they can do more than an average back. The reason that the position can be easily filled by any RB is that even the great RBs generally rush for 3 yards or less. The running game should really only be used for short yardage – and any RB can fill that role. Something I posted on another site:

My point is there is no running game, nor has there ever been a running game (or RB), that rattles off 4-5 yard gains at will. QB teams don’t hit every pass – but the great QBs are hitting 60%+ of their passes. RBs don’t come close to getting 4-5 yards 60% of the time.

The closest you are going to get to 60+% (that I can look up) is Jamaal Charles 2010 (who split time). He averaged 6.4 YPC and got 4+ yards 57% of the time.

Now, those career rushing bests, and all time NFL seasons.

Tom Brady, who most agree he had one of his worst statistical seasons of his career last year, including sacks, gained 4+yards on passes on ~50% of his attempts. One of Brady’s worst years passing is as effective or better than AD, LT, and Faulk’s best years rushing. Brady’s best year has him gaining 4+ yards on 60% of his pass plays. However, that doesn’t tell the whole story since a pass plays will generally gain more yards on average to begin with.

http://myweb.lmu.edu/ahealy Andrew Healy

I think that’s an awesome point. Here’s a potential explanation that still leaves RB easy to fill. Suppose a bunch of equally talented RBs get drafted at the same time. Who survives? The guy lucky enough to have a good line early on. Over time, though, his line ages and regresses (although not as quickly as him). So maybe the line is getting worse as RBs age since the ones that survive had better lines early on.

Kibbles

It could be that RBs are relatively fungible as long as they meet certain physical minimums (i.e. at least X speed, at least Y explosiveness, etc). That would explain why RB production is damn near flat from age 21 through 27- those guys meet the minimum, so year-to-year production swings are mostly just noise. Then, starting at age 28, RBs start to decline physically, and as soon as they no longer meet the physical minimums, their production falls through the floor.

Not saying that’s necessarily the best explanation, it’s just one possible theory that would explain both the “RBs are fungible” phenomenon and the “old RBs are less productive than young RBs, despite the fact that RBs are fungible” phenomenon.

Kibbles

Chase, what was the distribution of the declines among older RBs? For instance, your chart says that between age 29 and 30, RBs decline by 100 yards on average. Now, that could potentially mean two different things. It could mean that all RBs decline by a little bit (approximately 100 yards). Or it could mean that 80% of RBs don’t decline at all, but 20% of RBs fall off a cliff and decline by 500 yards. Either would explain the observed decline, but both carry very different implications. The first implies that all old RBs are certain to decline a little. The second implies that old RBs are simply a heightened risk to decline dramatically when compared to young RBs.

Unfortunately, there isn’t a perfect solution to the survivorship problem. One way to test how big it is would be to compare the YPC of the players in the study versus the YPC of the league as a whole. It’s almost certainly higher since we are only looking at the good, healthy players in N and N+1 and the average players that had a lucky, healthy N season and a healthy N+1 season, but presumably you care about aging for all RBs. The lucky N seasons might also mean you are OVERstating the decline, due to regression to the mean.

Another way to determine how big survivorship is: 1. How many players at each age (or what percentage) failed to hit 150+ carries at N+1? 2. What if you did the same analysis, but only looked at players that did NOT get 150+ carries in the next season? That might tell us if it’s primarily due to injury (good performance but few games) or ineffectiveness (bad performance), or at what age the switch occurs.

One option is to assume some sort of replacement level production from the players that fail to get to 150+ carries in N+1 and factor them in, and while you sort of assume your answer that way it might be most accurate. Another is to do away with the minimum carry limit and look at all players, but weight them according to the harmonic mean of carries for *both* seasons (harmonic mean where 1/x = 1/N + 1/(N+1)), but that minimizes effect of injury. You could also look at only players that had long careers, which will make a flatter curve, but give you a ‘minimum’ aging decline to contrast with the one above. You could also regress the N season performances and rerun the analysis, as the N seasons are likely good luck seasons (good luck seasons more likely to hit 150 carries in N and N+1 than bad luck seasons!).

If you do some or all of those you should get a variety of similar answers, and then combine them for one overall solution. Especially since one method’s weakness will be covered up by anothers!